1. Field of the Invention
This invention pertains to an antenna for radiating an omnidirectional antenna pattern. More specifically, it is directed to a coaxial type antenna having improved coupling, tuning and phasing characteristics.
Prior art techniques used to achieve onmidirectional coverage of a horizontally polarized signal have been somewhat complicated in implementation. Yet the need for a simpler antenna system to provide this type of pattern is present in applications such as television broadcasting.
To produce the required elevation pattern, the antenna designer calculates the phase and amplitude required in the numerous radiating elements along the vertical radiator. The radiating elements are then spaced at nominal 1/2 .lambda. or 1 .lambda. (where .lambda. is the wavelength) and each is matched to the line and so phased relative to adjacent radiating elements to give the required vertical pattern. The elements may be spaced around the vertical mast of the antenna to give an omnidirectional radiation in azimuth as may be required by the diameter to wavelength ratio of the coax, the larger diameter requiring more elements to obtain a required azimuth uniformity.
A common prior art antenna system which is easily adapted by the above described technique to give the desired pattern, utilizes a coaxial line as the vertical mast and vertical narrow slots in the outer conductor of the coaxial line as the individual radiator elements. Several methods have been used to couple the vertical slots to the transmission line. One is to couple the slots by means of inductive conducting loops behind the slots and oriented to align with the circumferential magnetic field within the coaxial line. A second coventional method of coupling the radiating slots to the transmission line is to simply connect one side of the slot to the center conductor of the coaxial line.
Both of the above coupling techniques provide a usable antenna covering one television channel in bandwidth.
Nonetheless the prior art antenna systems which have been used to achieve onmidirectional coverage with a horizontally polarized signal have required empirical adjustment by the antenna designer to achieve the desired antenna pattern, have been limited in frequency handling capability and the slot Q is often too high making it practically impossible to cover a 6 percent band as is usually required for carrying two television channels simultaneously near 800 MHz.